Probe light scanning actuator

ABSTRACT

Provided is an actuator for scanning detecting light, comprising an optical element for emitting detecting light, a moveable part supporting the optical element, a sheet spring having a fixed end and a moveable end supporting the moveable part; and drive means for driving the moveable part so as to scan the detecting light. Thus, a spring-mass system is formed in which the moveable part retaining the optical device acts as the mass, and the first order resonant frequency of the system may be selected so as to be higher than the operating frequency (scanning frequency). A bearing for a sliding part is not required, and the resistance loss can be thereby eliminated. These factors contribute to a favorable responsiveness. Also, by properly designing the sheet spring, a lighter and more compact design is enabled than would be possible with the conventional arrangement. A plurality of drive force generating units disposed on either side of the optical element in such a manner that the combined force of the drive force produced by the drive force generating units acts substantially onto the gravitational center of the moveable part. Thus, the drive efficiency can be improved while saving energy and achieving a high level of responsiveness.

TECHNICAL FIELD

[0001] The present invention relates to an actuator for scanning adetecting light beam which is suitable for use in a scanning device forscanning a detecting light beam such as a laser radar.

BACKGROUND OF THE INVENTION

[0002] Conventionally, various forms of scanning devices for scanning adetecting light beam are known, such as scan type laser radars, laserscanners, laser printers, laser markers and object monitoring devices.Among such devices, actuators for scanning a detecting light beam usedin scan type laser radars for preventing a vehicle crash include thoseimpinging a light beam onto a point on a polygonal mirror which isrotated by a motor and using the light beam reflected by the polygonmirror as a detecting light beam (FIG. 21), and those using a singlemoveable mirror turned or swung by a motor to reflect a light beamimpinged thereonto from a laser light source to scan the light beam byreflection as a detecting light beam (FIG. 22) (see Japanese patent laidopen publication No. 03-175390 and Japanese patent laid open publicationNo. 07-92270).

[0003] The polygon mirror type device illustrated in FIG. 21 includes apolygon mirror 31 which is rotatively driven by an electric motor 32,and a fixed reflective mirror 34 which directs a laser beam LB emittedfrom a laser diode 33 onto a point on the polygon mirror 31 so that thereflected laser beam may be scanned as the different reflective surfacesof the polygon mirror pass this point.

[0004] Such a polygon mirror type device is capable of a high speedscanning, but is both high is cost and large in size because a bearingis required for a sliding part that rotatably supports the mirror aswell as an electric motor for swinging or rotating the mirror.

[0005] The single mirror type device illustrated in FIG. 22 includes asingle moveable mirror 35 which is cyclically swung by an electric motor36. Laser light emitted from a laser diode 37 is impinged upon themoveable mirror 35 to scan the laser beam LB reflected by the moveablemirror. In this case, the moveable mirror is typically swung in a cyclicmanner by using a cam driven by a motor.

[0006] Such a single mirror type device is suitable for compact design,and costs less than a polygon mirror type device, but the need for abearing for rotatably supporting the mirror and an electric motor fordriving the mirror thereof prevents a reduction in cost. In particular,the need to rotate a single mirror prevents an increase in the scanningspeed. and the need to swing a single, mirror prevents achievement ofboth compact design and high frequency drive because of the problemsassociated with inertia and drive torque.

BRIEF SUMMARY OF THE INVENTION

[0007] To eliminate such problems, and provide an economical and compactactuator for scanning a laser beam capable of a high speed scanning, theactuator of the present invention comprises an optical element foremitting detecting light; a moveable part supporting the opticalelement; a sheet spring having a fixed end and a moveable end supportingthe moveable part; and drive means for driving the moveable part so asto scan the detecting light.

[0008] According to this arrangement, a spring-mass system is formed inwhich the moveable part retaining the optical device acts as the mass,and the first order resonant frequency of the system may be selectedhigher than the operating frequency (scanning frequency), A bearing fora sliding part is not required. and the resistance loss can be therebyeliminated. This contributes to a favorable responsiveness. The opticalelement for emitting detecting light is not limited to devices foremitting such light by themselves, but may also consist of any devicefor changing the light path of the detecting light emitted fromdetecting light emitting means to a desired direction.

[0009] The drive means may be provided with a plurality of drive forcegenerating units disposed on either side of the optical clement in sucha manner that the combined force of the drive force produced by thedrive force generating units acts substantially onto the gravitationalcenter of the optical element and moveable part. Thus. undesirablebehaviors resulting from an imbalance in moments can be avoided, and thedrive efficiency can be improved while saving energy and achieving ahigh level of responsiveness.

[0010] When the drive mean.- consists of an electromagnetic forcegenerating unit, an electromagnetic coil which is a relatively lightpart of the electromagnetic force generating unit may be provided on themoveable part so that the mass of the moveable part may be minimized.

[0011] The optical element may comprise a mirror for reflectingdetecting light emitted from laser light emitting means. The mirror mayconsist of a single mirror and a reflective surface thereof may be swungthrough a swinging motion of the sheet spring so that the scanning ofthe detecting light can be accomplished with a simple structure.

[0012] The optical element may comprise a prism for refracting detectinglight emitted from detecting light emitting means. In this case, theincident and exit angles of the detecting light into and out of theoptical element can be freely selected by appropriately designing theshape of the prism, and this contributes to the increase in the freedomin the layout and compact design of the actuator for scanning detectinglight.

[0013] A similar effect can be obtained even when the optical elementcomprises a hologram element for reflecting detecting light emitted fromdetecting light emitting means,

[0014] If the optical element comprises a detecting light emittingdevice, detecting light can be emitted directly from the moveable part,and the part surrounding the movable part can be made highly compactbecause there is no need for detecting light emitting means to beprovided outside the moveable part.

[0015] If the sheet spring is connected to a fixed part via a flexiblecircuit board including a circuit for supplying electric current to theelectromagnetic coil, the flexible circuit board provides a dampingaction to the sheet spring.

[0016] If the sheet spring is provided with a laminated structureincluding an electrically insulating layer and an electricallyconductive layer serving as. a circuit for supplying electric current tothe electromagnetic coil, the circuit for supplying electric current tothe electromagnetic coil can be formed at the same time as forming thesheet spring, and the wiring work is thereby simplified.

[0017] Also, by affixing 3 viscoelastic sheet or other vibration controlmaterial to a part of the sheet spring demonstrating a relatively highstrain at the time of resonance, the resonance property can be favorablycontrolled at low cost and without substantially increasing the mass ofthe system.

[0018] The drive mean may consist of an electromagnetic forcegenerating, and the sheet spring may comprise a plurality of sheetspring members disposed one next to another in a major plane of thesheet spring members with the electromagnetic force generating unitdisposed between the sheet .spring members. In this case. by arrangingthe sheet spring so that the electromagnetic force generating deviceacts substantially upon the gravitational center of the moveable part,and the drive force is applied substantially to the gravitational centerof the moveable part, it is possible to prevent undesirable behaviorsdue to the imbalance in moments from occurring. For instance, the numberof component parts and the mass of the core can be reduced and a morecompact and light-weight design is, made possible as compared to thearrangement in which a pair of electromagnetic force generating devicesare arranged above and below the single sheet spring member in asymmetric manner.

[0019] If each of the sheet spring members has a width which getsnarrower from the fixed end to the moveable end, the stress can bedistributed substantially uniformly over the sheet spring, and the spacefor accommodating the electromagnetic force generating unit can befavorably ensured.

[0020] If the electromagnetic force generating unit comprises anelectromagnetic coil attached to the moveable part while the coilreceives a supply of electric current via a circuit partly formed by thesheet spring members, the need for an extra wiring arrangement for theelectromagnetic coil of the moveable can be eliminated. Therefore, anyadverse effect such a wiring arrangement may have on the spring propertycan be avoided while the number of component parts can be reduced, andthe durability of the wiring arrangement can be improved.

[0021] If the electromagnetic force generating unit comprises a yokeattached to the fixed part, and the yoke includes a C-shaped memberwhich is folded onto itself to define a gap for receiving theelectromagnetic coil, the manufacturing process.% can be simplified.

[0022] Preferably, the electromagnetic coil is provided with an annularshape, and the yoke is attached to the fixed part so as to extend alongthe direction of movement of the movable part and partly fitted into theelectromagnetic coil, the fixed part being provided with a guide partfor guiding the yoke when fitting the yoke into the electromagnetic coilalong the direction of movement of the moveable part and attaching theyoke to the fixed part. This arrangement simplifies the assembling workfor the yoke.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Now the present invention is described in the following withreference to the appended drawings, in which;

[0024]FIG. 1 is a general block diagram of the scan type laser radarunit 1 for a vehicle crash prevention system embodying the presentinvention;

[0025]FIG. 2 is a schematic perspective front view showing an essentialpart of the scan unit 1 a;

[0026]FIG. 3 is a schematic perspective rear view showing an essentialpart of the,.s unit 1 a;

[0027]FIG. 4 is an exploded perspective view showing an essential partof the scan unit 1 a;

[0028]FIG. 5 is a vertical sectional view showing the moveable part;

[0029]FIG. 6 is a fragmentary perspective view of the sheet spring andan associated part using a flexible printed circuit board;

[0030]FIG. 7 is a diagram showing the relationship of the scanningfrequency to the fist order resonant frequency;

[0031]FIG. 8 is a vertical sectional view of the moveable part given asa modification of the first embodiment;

[0032]FIG. 9 is a fragmentary sectional view of the sheet spring givenas a modification of the first embodiment;

[0033]FIG. 10 is a cross sectional view of the sheet spring given as amodification of the first embodiment;

[0034]FIG. 11 is a view similar to FIG. 9 showing a modification of thefirst embodiment;

[0035]FIG. 12 is a diagram showing an embodiment using a prism as theoptical element;

[0036]FIG. 13 is a diagram showing an embodiment using a hologram as theoptical element;

[0037]FIG. 14 is a diagram showing an embodiment using a laser emittingdevice as the optical element;

[0038]FIG. 15 is a schematic perspective front view showing an essentialpart of the scan unit 21 a of a scan type laser radar unit 21 for avehicle crash prevention system given as a second embodiment of thepresent invention;

[0039]FIG. 16 is a schematic perspective rear view showing an essentialpart of the scan unit 21 a;

[0040]FIG. 17 is a plan view showing an essential pan of the scan unit21 a;

[0041]FIG. 18 is a cross sectional view taken along line XVIII-XVIII ofFIG. 17;

[0042]FIG. 19 is a perspective view showing the structure of the arcuateyoke 28 of the scan unit 21 a;

[0043]FIG. 20 is a view similar to FIG. 17 showing the mode ofassembling: the arcuate yoke 28 of the scan unit 21 a;

[0044]FIG. 21 is a schematic perspective view showing a conventionalpolygon mirror type laser actuator; and

[0045]FIG. 22 is a schematic perspective view showing a conventionalsingle mirror type laser actuator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] Now the present invention is described in the following in moredetail in terms of a concrete embodiment with reference to the appendeddrawings.

[0047]FIG. 1 is a block diagram of a scan type laser radar unit for avehicle crash prevention system embodying the present invention. Thescan type laser radar unit 1 is mounted, for instance, in a front partof a vehicle. The scan type laser radar unit 1 is incorporated with ascan unit 1 a consisting of a laser actuator, a scan unit controlcircuit 1 b for controlling the scan unit 1 a, a laser diode 1 c servingas a laser light emitting means for the scan unit 1 a, and a lightemitting element lighting circuit 1 d for controlling the laser diode 1e. The laser light beam from the laser diode 1 c is directed outward asa laser light beam scanned by the scan unit 1 a, and any reflecteddetecting light beam, for instance, reflected by an object ahead of thevehicle is received by a photodiode 1 f via a condenser lens 1 e.

[0048] The signal detected by the photodiode 1 f is amplified by anamplifying circuit 1 g and is then forwarded to a time measuring circuit1 h. The output signal of the time measuring circuit 1 h is forwarded toa measurement direction computing circuit 1 i. The measurement directioncomputing circuit 1 i is connected to a scan unit control circuit 1 b, alight emitting device lighting circuit 1 d, an amplifying circuit 1 kfor a smear detection sensor 1 j, a power source circuit 1 m and aninterface circuit 1 n. The interface circuit 1 n allows the measurementdirection computing circuit 1 i to exchange signals with other controlunits such as alarm displaying means and alarm sound emitting unit.

[0049]FIG. 2 is a schematic perspective front view of an essential partof the scan unit 1 a, and FIGS. 3 and 4 are schematic perspective rearview and exploded perspective view of the same, respectively.

[0050] Referring to these drawings, a rectangular post 3 stands uprighton a plate-shaped base 2 which is adapted to be attached to a casing ofthe scan type laser radar unit 1, and supports a base end of a sheetspring 4 having a major surface extending along the axial line of therectangular post 3. A free end of the sheet spring 4 remote from therectangular post 3 fixedly carries a moveable part consisting of amirror holder 5. The mirror holder 5 retains a mirror 6 extendingperpendicularly with respect to the major surface of the sheet spring 4.

[0051] The mirror 6 reflects the laser light from the laser diode 1 c tothe outside as shown in FIG. 1, and may consist of glass, plastic orlight metallic material such as aluminum. The surface (mirror surface)of the mirror 6 is provided with a reflective layer, for instance formedby depositing aluminum, having a smooth surface The surface of thereflective layer is coated by a protective layer consisting of SiO₂ orother thin film for the protection against corrosion and oxidization.

[0052] The mirror holder 5 fixedly carries thereon a pair ofelectromagnetic coils 7 a and 7 b forming a part of a drive meansconsisting of an electromagnetic force generating unit at upper andlower parts thereof (as seen in the drawing), respectively, by using abonding agent. The two electromagnetic coils 7 a and 7 b are disposedsymmetric about the center of the reflective surface of the mirror 6. aswell as about the gravitational center G of the moveable part whichincludes the mirror 6, mirror holder 5 and electromagnetic coils 7 a and7 b as shown in FIG. 5.

[0053] A pair of arcuate yokes 8 passing through the two electromagneticcoils 7 a and 7 b at upper and lower parts, respectively, and associatedyokes 9 each having a recessed part are integrally attached to yokebrackets 10 by threaded bolts each at either end thereof, and the yokebrackets 10 are in turn attached, by threaded bolts, to a yoke mountingpart 11 standing upright from the base 2. These yokes 8 and 9 may beformed by stamp forming soft magnetic member consisting of suchmaterials as pure iron.

[0054] A magnet 12 is fixedly attached to the recessed part of each ofthe arcuate yokes 9 opposite to the corresponding arcuate yoke 8.Therefore, a magnetic flux extends between each of the magnets 12 andthe corresponding arate yoke 8, and the corresponding electromagneticcoil 7 a or 7 b moves in the direction to cut the magnetic flux aselectric current is supplied to the electromagnetic coil 7 a or 7 b.Each of the electromagnetic coils 7 a and 7 b in this case consists ofapproximately 100 turns of copper wire wound without using a core. Thematerial. shape and dimensions of each of the magnets should be selectedappropriately so as to produce a required magnetic flux and produce anadequate drive force in cooperation with the correspondingelectromagnetic coil 7 a or 7 b. These components form the magneticcircuit.

[0055] The drive force produced by this magnetic circuit causes aswinging (scanning) motion to the mirror holder 5 (mirror 6) about thepivot point at which the sheet spring 4 is supported by the rectangularpost 3. Because the electromagnetic coils 7 a and 7 b are disposedsymmetrically about the gravitational center G of the moveable part asmentioned earlier, and the combined drive force of the electromagneticcoils 7 a and 7 b acts upon the gravitational center, the driveefficiency is improved, and undesirable behaviors which arc otherwisecaused by the imbalance in moments can be avoided. Also, because theelectromagnetic coils 7 a and 7 b which are relatively light among thecomponents of the electromagnetic force generating unit are provided onthe moveable part, the responsiveness and power efficiency can be bothimproved.

[0056] When molding the mirror holder 5 with plastic material, themirror 6 and electromagnetic coils 7 a and 7 b may be insert molded atthe same time. In such a case, the bonding process can be eliminated,and the production process can be favorably simplified. The mirrorholder 5 preferably consists of a light weight and high rigiditystructure as a moveable part, and therefore consists of a framestructure made by injection molding engineering plastic such as LCP(liquid crystal polymer) and FPS (polyphenylene sulfide) filled withglass fibers so as to define empty parts as illustrated in the drawings.

[0057] The sheet spring 4 may be formed by stamp forming a thin platemember made of beryllium copper, phosphorus copper or stainless steel.The shape of the sheet spring 4 is selected so as to make the firstorder resonant frequency of the moveable part (mirror holder 5) higherthan the scanning frequency, and control the stress of the sheet spring4 in use below the fatigue limit of the material. Thus, the durabilityof the sheet spring against repeated stress can be ensured.

[0058] The rectangular post 3 serving as a fixed part fixedly retainsthe sheet spring 4, and can be made by injection molding engineeringplastic such as LCP (liquid crystal polymer) and PPS (polyphcnylenesulfide) filled with glass fibers. The sheet spring 4 may be attached tothe rectangular post 3 by using a bonding agent or by mechanicallyattaching it to the rectangular post 3. Also, the sheet spring 4 andelectromagnetic coils 7 a and 7 b may be insert molded when injectionmolding the rectangular post 3 and mirror holder 5. In such a cage, thebonding process can be eliminated, and the production process can besignificantly simplified.

[0059] Referring to FIG. 6, a flexible printed circuit board 13comprising an electroconductive pattern 13 a for electrically connectingthe scan unit control circuit 1 b to the electromagnetic coils 7 a and 7b is bonded entirely over the sheet spring 4. A part of the flexibleprinted circuit board 13 is bent into a crank shape, and is fixedlyattached to the rectangular post 3 by using a bonding agent. Because therectangular post 3 and sheet spring 4 are attached to each other via aresilient member (a part of the flexible printed circuit board 13), themoveable part can be given with an appropriate damping action bysclecting the material, thickness and shape of the flexible printedcircuit board 13. Therefore, the electromagnetic coils 7 a and 7 b arenot required to produce a large braking force at each point of reversingthe swinging motion, and this contributes to the saving of the electricpower consumption and the increase in responsiveness.

[0060] A stopper 19 made of resilient material such as rubber isattached to each of the yoke brackets 10 by using a bonding agent orother fastening means to limit the scaning angle (θ) of the moveablepart in case an excessive current is supplied to the electromagneticcoils 7 a and 7 b. When the moveable part (such as the mirror 6) hasrotated more than is required. each of the stoppers 19 engages a part ofthe corresponding mirror holder 5, and limits the maximum rotationalangle thereof. Thus, the moveable part is prevented from swingingexcessively.

[0061] According to the scanning type laser radar unit 1 incorporatedwith the scan unit 1 a described above, the laser diode 1 c consists ofa near infrared (having a wavelength in the order of 900 nm) pulse laserdiode, and produces light pulses each having a duration in the order ofa few μm according to the control signal from the light emitting devicelighting circuit 1 d. The laser light from the laser diode 1 c isreflected by the mirror 6 of the scan unit 1 a, and is emitted to theoutside as a laser beam LB.

[0062] The two electromagnetic coils 7 a and 7 b receive a supply ofelectric current corresponding to the control signal from the scan unitcontrol circuit 1 b. and the mirror holder 5 (mirror 6) swings about theaxial line of the rectangular post 3 according to the polarity andamplitude of the electric current. As the angle of the reflectivesurface of the mirror 6 changes, the laser beam LB emitted to theoutside as a reflected light beam undergoes a scanning or sweepingaction. The electric current typically consists of an alternatingcurrent having a frequency in the order of 30 Hz.

[0063] An arcuate sensor 20 extends between the two yoke brackets 10 todetect the origin and angle of the minor holder 6, and a correspondingsensing plate 20 a is attached to the lower electromagnetic coil 7 b,for instance. The sensor 20 provides such information as the swing angle(scanning angle), angular position (absolute value), angular speed andoperating frequency. Therefore, both the distance information (which canbe computed from the state of the received pulsed laser beam) andangular information can be obtained, and it can be used for accuratelydetecting the position of a vehicle which could cause a vehicle crashThe sensor 20 and sensing plate 20 a may consist of a contact-lessoptical or magnetic encoder.

[0064] The scan unit 1 a serving as a laser actuator forms a spring-massswing, and is given with a first order resonant frequency f₀ which ishigher than the operating frequency (scanning frequency) f_(s) toachieve a favorable responsiveness as shown in FIG. 7.

[0065] A modification of the first embodiment is described in thefollowing. FIG. 8 is a view similar to FIG. 5 showing the modificationof the first embodiment, and the description of the remaining parts isomitted as they are similar to those of the previous illustratedembodiment.

[0066] In this example, a film strip 4 c for damping made of polymermaterial having a damping property is affixed to a part of the sheetspring 4 which is subjected to a relatively large strain at the time ofthe second-order resonant vibration mode of the sheet spring 4. Thisposition is typically found near the free end of the sheet spring 4 orthe upper and lower positions near the electromagnetic coils 7 a and 7b, and the size, position and material of the film strip 4 c areselected so that a desired damping effect is produced. This controls theresonance peak of the second order resonance mode as indicated by thesolid line in FIG. 7, and prevents the destruction of the device due toresonant vibrations and the induction of spurious vibrations duc toexternal interferences. Also, the electromagnetic coils 7 a and 7 b arenot required to produce a large braking force at each point of reversingthe swinging motion so that the energy consumption can be minimized, anda high level of responsiveness can be achieved. The imaginary line onthe right hand side of FIG. 7 shows the response that is produced whenthe film strip 4 c is not affixed.

[0067] When a vibration control member is attached to the sheet spring 4near the base end thereof, the first-order resonance mode can be alsocontrolled as indicated by the imaginary line in the central part (f₀)of FIG. 7. Instead of using a film strip 4 c having a damping property,viscoelastic material in the form of gel may be applied to anappropriate part of the sheet spring. By thus selectively applying avibration control member to a suitable part, a desired frequencyresponse can be obtained without increasing the mass of the system.

[0068] Another modification of the first embodiment is described in thefollowing. FIG. 9 is a schematic perspective view of an essential partof the sheet spring given as a modification of the first embodiment, andthe remaining parts are omitted from the description as they may besimilar to those of the previous illustrated embodiment.

[0069] According to this embodiment, the sheet spring 4 of the previousembodiment is divided into upper and lower parts 4 a and 4 b, one end ofeach of the two parts 4 a and 4 b of the sheet spring is supported bythe rectangular post 3 independently, and a mirror holder 5 is supportedby the other ends of the two parts of the sheet spring 4. The sheetspring 4 may be likewise formed by stamp forming a thin plate membermade of beryllium copper, phosphorus copper or stainless steel, and canbe used as electric leads for the electromagnetic coils 7 a and 7 b.This simplifies the electric current supply circuit. The shape of thesheet spring 4 is selected so that the first order resonant frequency ofthe moveable part (the part of the mirror holder 5) is higher than thescanning frequency and the stress of the spring in operation is belowthe fatigue limit.

[0070] The sheet spring parts 4 a and 4 b should be given with a dampingaction by using suitable means. For instance, a polymer film striphaving a damping property may be attached to the sheet spring parts 4 aand 4 b or a viscoelastic member in the form of gel may be applied tothe sheet spring parts. If a sensor signal is required to be obtainedfrom the moveable part, a flexible printed circuit board may be used. Insuch a case, the flexible printed circuit board may be given with adamping capability, and an additional damping structure may be omitted.

[0071]FIG. 10 is a cross sectional view of yet another modification ofthe sheet spring given as a third embodiment of the present invention.In this embodiment, the sheet spring consists of a thin sheet spring 14and forms a three layer structure by additionally comprising anelectrically insulating layer 15 formed on the thin sheet spring 14, andan electroconductive layer 16 for supplying electric current to theelectromagnetic coils 7 a and 7 b and a sensor signal conducting layer17 for conducting electric current for a sensor provided on the moveablepart both formed on the electrically insulating layer 15. Thisarrangement eliminates the need to form an electric current find circuitusing lead wire, and the wiring work can be simplified. If the sensingplate is formed on the moveable part as was the case with the previousembodiments, the sensor signal conducting layer 17 would not berequired.

[0072]FIG. 11 is a view similar to FIG. 9 showing yet anothermodification of the first embodiment of the present invention, and thesheet spring consists of a thin sheet spring 14. A five layer structureis formed by a pair of electrically insulating layers 15 formed oneither side of the thin sheet spring 14, and an electroconductive layer16 and a sensor signal conducting layer 17 formed on the surface of eachelectrically insulating layer 15. This embodiment provides a similareffect as that of the embodiment shown in FIG. 10. As compared to thethree layer structure, the number of conductive paths can be readilyincreased and a finer control of the electromagnetic coils 7 a and 7 bis enabled by individually controlling the electric current supplied toeach of the coils. it is also possible to increase the number ofelectromagnetic coils 7 a and 7 b to four, twice that of the illustratedembodiment. Also, according to the five layer structure, because thelayers are arranged symmetrically about the thin sheet spring 14, a morefavorable balancing of the spring property and damping property of thethin sheet spring is enabled as compared to the three layer structure.

[0073] The electrically insulating layers 15. electroconductive layer 16and sensor signal conducting layer 17 may be formed by etching, pressingand punching appropriate materials similar to those used for theflexible circuit board 13 in a manner suitable for mass production.

[0074] The thin sheet springs 14 used in the embodiments illustrated inFIGS. 10 and 11 may be made by pressing and -punching thin springmaterial such as beryllium copper, phosphorus copper or stainless steel,and the shape of the sheet spring 14 is selected so as to make the firstorder resonant frequency of the moycabic part (mirror holder 5) higherthan the scanning frequency, and control the stress of the sheet springin use below the fatigue limit of the material.

[0075] Referring to FIG. 12, a moveable holder 15 may be provided tosupport a prism 16 instead of the mirror holdcr 5 which supported themirror 6 in the foregoing embodiments. This arrangement also allows thelight emitted from the laser diode 1 c to project a laser beam LB in asimilar manner as those of the foregoing embodiments via the prism 16.At the same time, the moveable holder 15 (the prism 16) is made toundergo a swinging motion as indicated by arrow A to scan the laser beamLB. This embodiment also provides a similar effect. Because the swingingangle of the moveable holder 15 can be relatively reduced for the givenscanning angle θ that is required for the laser beam LB emitted via theprism 16. Therefore, the laser beam LB can be scanned by using arelatively small power, and this contributes to a compact design and alower power consumption.

[0076] Referring to FIG. 13, a hologram device 17 may be used instead ofthe prism 16 shown in FIG. 12. The parts corresponding to those of theembodiment shown in FIG. 12 are denoted with like numerals withoutrepeating the description of such parts. This arrangement also allowsthe light emitted from the laser diode 1 c to project a laser beam LB ina similar manner as those of the foregoing embodiments via the hologramdevice 17. At the same time, the moveable holder 15 (the hologram device17) is made to undergo a swinging motion as indicated by arrow A to scanthe laser beam LB. This embodiment also provides a similar effect.

[0077] Referring to FIG. 14, a light emitting device 18 such as a laserdiode serving as a laser light emitting device may be provided on themoveable holder 15, instead of the prism 16 used in the embodiment shownin FIG. 12. In this case. a laser beam LB is directly emitted from thelight emitting device 1.8 of the moveable holder 15. Al the same time,the moveable holder 15 (the light emitting device 18) is made to undergoa swinging motion as indicated by arrow A to scan the laser beam LB.This embodiment also provide a similar effect, and allows the partsurrounding the movable part to be made both simple and compact withoutacquiring the laser diode 1 c to be provided outside the scan unit 1 a.

[0078]FIG. 15 is a schematic perspective front view of an essential partof the scan unit 21 a of a scan type laser radar unit for a vehiclecrash prevention system, and FIG. 16 is a schematic perspective rearview of the same. FIG. 17 is a plan view of the same, and FIG. 18 is asectional side view taken long line XVIII-XVIII of FIG. 17. As theoverall structure of the a scan type laser radar unit for a vehiclecrash prevention system is similar to that of the first embodiment, theillustration and description of the same are omitted. As shown in thesedrawings. a fixed part 23 having a C-shaped side view is attached to abase 22 which is adapted to be attached to a casing of a scan type laserradar unit 1, for instance. One end (base end) of each of a pair ofupper and lower sheet spring members 24 a and 24 b is attached to thefixed part 23. Three or more sheet spring members may be used inpractice, but it is preferable to determine the number so that theelectromagnetic force generating unit may be located substantially atthe gravitational center of the moveable part. The sheet spring members24 a and 24 b are arranged in parallel to each other so that their majorsurfaces are located on a same plane. The other ends or moveable ends ofthe sheet spring members 24 a and 24 b fixedly support a mirror holder25 serving as a retaining part. The mirror holder 25 retains a singlemirror 26 Serving as an optical element in such a manner that the majorsurface of the mirror 26 extends perpendicularly to the major surface ofthe sheet spring members 24 a and 24 b. Each of the sheet spring member24 a and 24 b is provided with a width which gets progressively narrowerfrom the fixed end (base end) toward the other or moveable end thereof.Thereby, the stress of each of the sheet spring members functioning as acantilever can be distributed evenly within the sheet spring member, andthe space for accommodating the electromagnetic force generating unitcan be ensured in an efficient manner. Because the sheet spring members24 a and 24 b arranged above and below the electromagnetic forcegenerating unit in a spaced relationship, the rigidity against rollingmotion can be improved making the assembly less susceptible to externalinfluences.

[0079] The mirror 26 reflects the detecting light beam from the laserdiode 21 c to the outside as shown in FIG. 1, and may consist of glass,plastic or light metallic material such as aluminum. The surface (mirrorsurface) of the mirror 26 is provided with a reflective layer, forinstance formed by depositing aluminum, having a smooth surface. Thesurface of the reflective layer is coated by a protective layerconsisting of SiO₂ or other thin film for the protection againstcorrosion and oxidization.

[0080] A pair of electromagnetic coils 27 a and 27 b forming anelcctromagnetic force generating unit serving as a drive means areattached to the mirror holder 25 in such a manner that the drive forcemay act substantially upon the gravitational G of the moveable partwhich includes the electromagnetic coils 27 a and 27 b, mirror holder 25and mirror 26.

[0081] An arcuate yoke 28 having a lower part 28 a which is fitted intothe electromagnetic coils 27 a and 27 b is attached to the base 22. Thearcuate yoke 28 comprises a lower part 28 a and an upper part 28 b whichare spaced from each other and formed by bending a substantially annularmember provided with a bending section 28 c. A magnet 29 is fixedlyattached to the surface of the upper part 28 b facing the lower part 28a. The free ends of the upper and lower parts 28 a and 28 b remote fromthe bending section 28 c are attached to a yoke retaining part 22 a byinterposing the same and passing fasteners made of magnetic materialsuch as threaded bolts through them so ;e to define a magneticallyclosed structure. Therefore, a magnetic flux extends between the magnet29 and the lowcr part 28 a of the arcuate yoke 28, and a drive force isproduced by the electromagnetic coils 27 a and 27 b in the direction tocut across the magnetic flux as electric current is supplied to theelectromagnetic coils 27 a and 27 b.

[0082] In practice, the upper and lower parts 28 a and 28 b of thearcuate yoke 28 can also be formed by joining one ends a pair ofsemicircular members, and bending the assembly into two halves. By thusforming the lower part 28 a and upper part 28 b in advance, thepositional ends of the two parts can be accommodated to a certain extentduring the fabrication process, and the assembly work can be simplifiedwhile reducing the required number of component parts as compared to thecase where the lower part 28 a and upper part 28 b are positioned as aseparate member when assembling the scan unit 21 a.

[0083] The drive force produced by this magnetic circuit causes aswinging (scanning) motion to the mirror bolder 25 (mirror 26) about thepivot point at which the sheet spring members 24 a and 24 b aresupported by the rectangular post 23. Because the drive force acts uponthe gravitational center G of the moveable part s mentioned earlier, thedrive efficiency is improved, and undesirable behaviors which areotherwise caused by the imbalance in moments can be avoided. Also,because the electromagnetic coils 27 a and 27 b which arc relativelylight among the components of the electromagnetic force generating unitare provided on the moveable part, the responsiveness and powerefficiency can be both improved.

[0084] As shown in FIG. 15, electrode terminals 24 c and 24 d integralwith the sheet spring members 24 a and 24 b project from the back sideof the fixed part 23. These electrode terminals 24 c and 24 d areconnected to the scan unit control circuit 21 b and the free ends(moveable ends) of the sheet spring members 24 a and 24 b are connectedto the electromagnetic coils 27 a and 27 b so that electric currant issupplied to the electromagnetic oils 27 a and 27 b via the electrodeterminals 24 c and 24 d and sheet spring members 24 a and 24 b.Therefore. there is no need to attach an additional wiring member suchas a flexible printed circuit board to the sheet spring members 24 a and24 b, and the impairment of the responsiveness and generation ofunwanted vibration modes due to the additional rigidity provided by sucha wiring member can be avoided. Because the electrode terminals 24 c and24 d are very small and thermally highly conductive, the soldering workfor the connection can be conducted in an efficient manner.

[0085] The sheet spring members 24 a and 24 b may be formed by stampforming or etching a thin plate member made of beryllium copper,phosphorus copper or stainless steel.

[0086] Viscoelastic material is bonded to the surfaces of the sheetspring members 24 a and 24 b to provide an appropriate damping action tothe moveable part. Therefore, a damage to the device due to resonancecan be avoided, and spurious vibrations due to external disturbances canbe avoided. Also, the electromagnetic coils 27 a and 27 b are notrequired to produce a large braking force at each point of reversing theswinging motion, and this contributes to the saving of the electricpower consumption and the increase in responsiveness.

[0087] The fixed part 23 and mirror holder 25 may be made by injectionmolding engineering plastic which is both light and rigid such as LCP(liquid crystal polymer) and PPS (polyphcnylcnc sulfide) filled withglass fibers.

[0088] The sheet spring members 24 a and 24 b may be attached to thefixed part 23 and mirror holder 25 after these parts are molded.Alternatively, the sheet spring members 24 a and 24 b may be integrallyformed with the fixed part 23 and mirror holder 25 by using the sheetspring members 24 a and 24 b as insert members. By so doing, thepositional precision can be improved as compared to the case where thesheet spring members 24 a and 24 b are attached to the fixed part 23 andmirror holder 25 after these parts are molded. It is also possible toform the two sheet spring members as a one-piece member provided with aconnecting part (not shown in the drawing) by stamp forming or etching,and remove this connecting part after the fixed part 23 and mirrorholder 25 are integrally molded with the sheet spring members 24 a and24 b.

[0089] Referring to FIG. 20, the mode of assembling the scan unit 21 ais described in the following. First of all, the fixed part 23, sheetspring members 24 a and 24 b and mirror holder 25 are fixedly attachedto each other, and the electromagnetic coils 27 a and 27 b are fixedlyattached to the mirror holder 25 by using a bonding agent. The fixedpart 23 is then attached to the base 22. The lower part 28 a of thearcuate yoke 28 is fitted into the electromagnetic coils 27 a and 27 b,and the yoke 28 is fixedly attached to the yoke retaining part 22 a ofthe base 22 with the yoke retaining part 22 a interposed between thelower part 28 a and upper part 28 b and the end surfaces thereofabutting the stopper surface 22 b of the bass 22. At this point, becausethe base 22 is provided with guide surfaces 22 e and 22 d for guidingthe inner circumferential surface of the arcuate yoke 28, the assemblingand positioning work of the arcuate yoke 28 is simplified.

[0090] According to the scanning type laser radar unit 21 incorporatedwith the scan unit 21 a described above, the laser diode 21 c consistsof a near infrared (having a wavelength in the order of 900 nm) pulselaser diode, and produces light pulses each having a duration in theorder of a few ∥m according to the control signal from the lightemitting device lighting circuit 21 d. The detecting light from thelaser diode 21 c is reflected by the mirror 26 of the scan unit 21 a,and is emitted to the outside as a detecting light beam LB.

[0091] The two electromagnetic coils 27 a and 27 b receive a supply ofelectric current corresponding to the control signal from the scan unitcontrol circuit 21 b, and the mirror holder 25 (mirror 26) swings aboutthe axial line of the rectangular post 23 according to the polarity andamplitude of the electric current. As the angle of the reflectivesurface of the mirror 6 changes, the detecting light beam LB emitted tothe outside as a reflectcd light beam undergoes a scanning or sweepingaction. The electric current typically consists of an alternatingcurrent having a frequency in the order of 30 Hz. This alternatingcurrent may be PWM controlled if necessary.

[0092] The scan unit 21 a serving as a laser actuator forms aspring-mass swing, and is given with a first order resonant frequency f₀which is higher than the operating frequency (scanning frequency) f_(s)to achieve a favorable.

[0093] Although the optical element consisted of a mirror in theforegoing embodiment, the optical element may also comprise a prism. Inthis case, because the swinging angle of the moveable holder can berelatively reduced for the given scanning angle θ that is required forthe detecting light beam LB emittcd via the prism, the detecting lightbeam LB can be scanned by using a relatively small power. Thiscontributes to a compact design and a lower power consumption. Theoptical element may also consist of a hologram device. It is alsopossible that the optical element comprises a laser light emittingdevice. In such a case. a detecting light bcam LB can be emittcddirectly from the light omitting device, and the part surrounding themovable part can be made highly compact because there is no need forlaser beam emitting means to be provided outside the scan unit 1 a.

[0094] Thus, according to the present invention, a spring-mass system isformed in which the moveable part retaining the optical device acts asthe mass, and the first order resonant frequency of the system may beselected so as to be higher than the operating frequency (scanningfrequency) A bearing for a sliding part is not required. and theresistance loss can be thereby eliminated. These factors contribute to afavorable responsiveness. Also, by properly designing the sheet spring,such as controlling the bending stress of the sheet spring which arisesduring operation below the fatigue limit, a lighter and more compactdesign is enabled than would be possible with the conventionalarrangement using a polygon mim)r. Becausc the moveable part is directlyactuated while requiring fewer component parts, and the structure issimplified, a highly low cost design is possible.

[0095] By providing a plurality of drive force generating devicessymmetrically about the optical device, and coinciding the compositedrive force with the gravitational center of the moveable part, thedrive efficiency can be improved while saving energy and achieving ahigh level of responsiveness. An electromagnetic coil which is arelatively light part of the electromagnetic foroo generating unit maybe provided on the moveable part so that the mass of the moveable partmay be minimized. The optical clement may comprise a mirror forreflecting detecting light emitted from laser light emitting means. Themirror may consist of a single mirror and a reflective surface thereofmay be swung through a .swinging motion of the sheet spring so that thescanning of the detecting light can be accomplished with a simplestructure.

[0096] By using a prism as the optical device, the swing angle of themoveable part can be reduced for a given swing angle of the detectinglight, and the required scanning of the detecting light can be achievedby using a relatively small drive force. By using a hologram device asthe optical device, a similar result can be achieved. By using a laserlight emitting device as the optical device, the detecting light can beproduced directly from the moveable part, and the need for an externallaser light emitting means can be eliminated. This allows a compactdesign of the part surrounding the moveable part.

[0097] By connecting the sheet spring to the fixed part via a flexiblecircuit board for supplying electric current to the electromagneticcoils, a damping action can be produced from the flexible circuit board,and the responsiveness can be improved. In particular, by laminating anelectrically insulating layer and an electroconductive layer forsupplying electric current to the electromagnetic coils, theelectroconductive circuit for supplying electric current can be formedat the same time as forming the sheet spring, and the assembly work suchas wiring can be simplified.

[0098] Also, by affixing a viscoelastic sheet or other vibration controlmaterial is applied to a part of the sheet spring demonstrating arelatively high strain at the time of resonance, the resonance propertycan be favorably controlled at low cost and without substantiallyincreasing the mass of the system.

[0099] If the drive means consists of an electromagnetic forcegenerating unit, and the sheet spring comprises a plurality of sheetspring members disposed one next to another in a major plane of thesheet spring members with the electromagnetic force generating unitdisposed between the sheet spring members in such a manner that amassive part of the moveable part is concentrated near the driving pointthereof and the drive force acts substantially upon the gravitationalcenter of the moveable part, it is possible to prevent undesirablebehaviors duc to the imbalance in monuments from occurring. Because thetwo sheet spring members are spaced from each other, a rigidity againstrolling motion can be improved. Also, the number of component parts andthe mass of the core can be reduced and a more compact and light-weightdesign is made possible as compared to the arrangement in which a pairof electromagnetic force generating devices are arranged above and belowthe single sheet spring member in a symmetric manner. Because each ofthe sheet spring mcmbcrs has a width which gets narrower from the fixedend to the moveable end, the stress can be distributed substantiallyuniformly over the sheet spring, and the space for accommodating theelectromagnetic force generating unit can be favorably ensured.

1. An actuator for scanning detecting light. comprising: an opticalelement for emitting detecting light; a moveable part supporting theoptical element; a sheet spring having a fixed end and a moveable endsupporting the moveable part; and drive means for driving the moveablepart so as to scan the detecting light.
 2. An actuator for scanningdetecting light according to claim 1, wherein the drive means isprovided with a plurality of drive force generating units dispos.ed oneither side of the optical element in such a manner that the combinedforce of the drive force produced by the drive force generating unitsacts substantially onto the gravitational center of the optical elementand moveable part.
 3. An actuator for scanning detecting light accordingto claim 1, wherein the drive means consists of an electromagnetic forcegenerating unit and the moveable part comprises an electromagnetic coil.4. An actuator for scanning detecting light according to claim 1,wherein the optical clement comprises one of a member selected from agroup consisting of a mirror for reflecting detecting light emitted froma laser light emitting mans, a prism for refracting detecting fightemitted from a laser light emitting means, a hologram element forreflecting detecting light emitted from a laser light emitting means,and a detecting light emitting device.
 5. An actuator for scanningdetecting light according to claim 3, wherein the sheet spring isconnected to a fixed part via a flexible circuit board including acircuit for supplying electric current to the electromagnetic coil. 6.An actuator for scanning detecting light according to claim 3, whereinthe sheet spring is provided with a laminated structure including anelectrically insulating layer and an electrically conductive layerserving as a circuit for supplying electric current to theelectromagnetic coil.
 7. An actuator for scanning detecting lightaccording to claim 3, wherein a vibration control member is affixed tothe sheet spring at a part where a relatively large strain is producedin a resonant vibration.
 8. An actuator for scanning detecting lightaccording to claim 1, wherein the drive means consists of anelectromagnetic force generating unit for driving the moveable, and thesheet spring comprises a plurality of sheet spring members disposed onenext to another in a major plane of the sheet spring members, theelectromagnetic force generating unit being disposed between the sheetspring members.
 9. An actuator for scanning detecting light according toclaim 8, wherein the each of the sheet spring members has a width whichgets narrower from the fixed end to the moveable end.
 10. An actuatorfor scanning detecting light according to claim 8, wherein theelectromagnetic force generating unit comprises an electromagnetic coilattached to the moveable part, the coil receiving a supply of electriccurrent via a circuit partly formed by the sheet spring members.
 11. Anactuator for scanning detecting light according to claim 8, wherein theelcctromagnetic force generating unit comprises a yoke attached to thefixed part, the yoke including a C-shaped member which is folded ontoitself to define a gap for receiving the electromagnetic coil.
 12. Anactuator for scanning detecting light according to claim 8, wherein theelectromagnetic coil is provided with an annular shape, and the yoke isattached to the fixed part so as to extend along the direction ofmovement of the moveable part and partly fitted into the electromagneticcoil, the fixed part being provided with a guide part for guiding theyoke when fitting the yoke into the electromagnetic coil along thedirection of movement of the moveable part and attaching the yoke to thefixed part.
 13. An actuator for scanning detecting light according toclaim 8, wherein the optical element comprises a member selected from agroup consisting of a mirror for reflecting detecting light emitted froma detecting light omitting means; a prism or lens for changing theoptical direction of detecting light emitted from a detecting lightemitting means; a hologram for reflecting detecting light emitted from adetecting light omitting means; and a detecting light emitting meansitself.